1. Field of the Invention
The present invention relates to a manufacturing process for color plasma display panels (PDPs), and more particularly to a pretreatment for forming phosphor layers.
2. Description of the Related Art
In a color plasma display panel, gas discharge generates ultraviolet ray to excite three different color phosphor layers respectively coated inside of discharge cells to emit red, green and blue lights for achieving color displaying. FIG. 4 shows a typical panel structure of surface discharge type AC color plasma displays. Inner surface of a front glass substrate 6 is provided with surface discharge electrodes 7, each consisting of transparent conducting films formed by stacking metallic bus electrodes. A transparent dielectric layer or glaze layer 8, is coated on the electrodes 7 and a black matrix 9 is formed on the glaze layer 8 so as to determinate pixels. On a glass substrate 1 on the back side are formed data electrodes 2, a glaze layer 3, and striped white barriers 4. Phosphor 5, alternately colored in red, green and blue, is disposed over the side faces of the white barriers 4 and groove bottoms between them, which together constitute a discharge cells.
Discharge gas is sealed into the space between the two glass substrates to complete a panel. To scanning electrodes are successively applied scanning pulses and, in synchronism with them, data pulses are applied to selected data electrodes. After this line-by-line successive scanning has covered the whole panel, sustaining discharge is caused to take place all over the panel to achieve color luminescence. This operation is accomplished in a plurality of subfields each having a prescribed frequency of luminescence corresponding to digitized gradation data in a field period of 1/60 of a second to display color pictures of television of the like.
In the manufacturing process of such a panel, the process of forming finely patternized phosphor layers is of critical importance. Where a phosphor coat is to be formed over a flat substrate, with no protruding barriers on it, a satisfactory layer can be formed by exposing to light and developing phosphor patterns, each corresponding to one or another of the three primary colors.
However, this method cannot be readily applied where barriers are formed and a phosphor layer has to be formed on the side surfaces of the barriers, too, as in the structure illustrated in FIG. 4. To such a case, a screen printing process is applied, whereby a screen having striped openings of three times as high a pitch as the striped cells are used, and the inner surface of the cells are coated through the screen meshes with a paste containing a binder and a solvent. This process is repeated with a drying process in-between to form phosphor layers of three colors. Apart from this method, application with a fine dispenser is also proposed.
Whereas the pixel size of a color plasma display varies with the screen size and the use, the pitch of barriers ranges from 130 to 500 microns for a television or personal computer monitor panel measuring from 20 to 60 inches diagonally, i.e. the main applicable range of color plasma displays. The barriers are from 100 to 200 microns high and from 30 to 100 microns wide, and this means that the phosphor layers need to be formed over the bottom of discharge cells and the side faces of barriers of a high aspect ratio, which make up narrowly limited spaces between them.
The bottom surface of the discharge cells is a high dense structure, such as a glass substrate, a metallic electrode or a glaze layer. The barriers are finely processed by applying a thick film processing technique, such as sand blasting, to a paste layer consisting of a mixture of the powder of an oxide, such as alumina, and glass having a low melting point, and firing the product of this process at high temperature. As the glass content is kept low to minimize the deformation by firing, the barrier portion is often porous. Though it is conceivable to simplify the process by firing the barriers at a time after their coating with phosphor, but the barriers portion not yet fired but only dried is highly porous and accordingly not strong enough.
Since the structure to which phosphor paste is to be applied is variable in absorptiveness, surface roughness and other respects, the state of coating is made all the more susceptible to unevenness between the bottom and the barrier sides. Moreover, the shape of the phosphor coat may differ with the sequence of phosphor application. While discharge cells on both sides of the first applied phosphor are uncoated with phosphor, the second and third find phosphor coating on either side and both sides, respectively. The influence of this state of the adjoining cells is particularly significant when the barriers are porous, and the sequence of coating may give rise to uneven presence of phosphor or a difference in the amount of coating between the barrier sides and the bottom.
FIG. 5 schematically illustrates the cross-sectional shapes of a phosphor coat 5 over a substrate 1 having barriers 4. While FIG. 5A shows a satisfactory state, FIG. 5B shows coating of only the bottom, and FIG. 5C, an extreme case in which phosphor is lumped over the middle part of the bottom. In the example of FIG. 5D, conversely, the sides of the barriers 4 are thickly, but their bottom is scarcely, coated. Uneven coating, such as the case of FIG. 5E, would involve the trouble of substantial fluctuations in brightness when the screen is looked at obliquely.
These uneven distributions of phosphor would affect not only the drive performance of the color plasma display but also the distributions of tint, brightness and visual angle-dependence over the area of the panel. Since human sight is sensitive to these distributions, it is of vital importance to achieve a sufficiently uniform state of phosphor coating for the three primary colors over the whole area of the panel display screen.
Delicate adjustment and sophisticated coating control of phosphor paste have been attempted only to invite a drop in the yield and a rise in the cost of panel manufacture. Firing of the barrier portion together with the phosphor coat, instead of firing it in advance, has an advantage for the manufacturing process as it serves to simplify the process and makes possible protection of the substrate from thermal deformation during the barrier firing, but this simplified process has been prevented from practical use by its inability to apply phosphor with a satisfactory result.